The present invention relates to heat vulcanizable silicone rubber compositions and more particularly the present invention relates to the removable of volatiles from heat vulcanizable silicone rubber compositions.
Silicone elastomeric compositions are well known. Basically, there are two types of such compositions, room temperature vulcanizable silicone rubber compositions and heat vulcanizable silicone rubber compositions. The room temperature or RTV compositions comprise the one component type and the two component type. The two component type comprise a silanol diorganopolysiloxane polymer, optionally a filler, an alkyl silicate as a cross-linking agent and a metal salt of a carboxylic acid as the catalyst. The composition is packaged such that the alkyl silicate is packaged separately from the silanol polymer and when it is desired to cure the composition, the two packages are mixed and applied to cure to a silicone elastomer with a complete cure taking place in anywhere from 24 to 72 hours.
Then, there is another type of silicone RTV silicone rubber composition and specifically, the one component RTV composition. Such one component RTV composition comprises a silanol diorganopolysiloxane polymer as a base polymer, optionally a filler, an alkyoxy or acetoxy functional silane as a cross-linking agent and a metal salt or a carboxylic acid in the case of an acyloxy cross-linking agent and a titanium chelate catalyst in the case of the alkoxy cross-linking. In a one component system all the ingredients are mixed in a substantially anhydrous state. When it is desired to cure the composition, the seal on the package is broken and the composition is applied and exposed to atmospheric moisture whereupon due to the exposure to atmospheric moisture, the cross-linking agent hydrolyzes and cross-links the composition to a silicone elastomer with a complete cure taking place in anywhere from 24 to 72 hours. The advantage of the one component as distinguished from the two component system is that the two component system requires mixing on site to properly mix and prepare the composition when applying it to the form that is desired. Both of these compositions are distinguished by the fact that they cure to a silicone elastomer at room temperature.
There is another type of composition that cures at either room temperature or at elevated temperatures. This composition comprises a vinyl containing diorganopolysiloxane polymer, a hydride silicone resin, or a hydride polysiloxane cross-linking agent and a platinum catalyst as the catalyst. Normally such a composition is packaged in two components and is mixed in the proper ratios to cure to a silicone elastomer either at room temperature or very quickly at elevated temperatures. Such compositions which are called addition cured systems may utilize inhibitors such as that may be packaged in a single package at room temperature for long periods of time but will cure very rapidly at elevated temperatures, upon heating above 100.degree. C. Such cured systems are not particularly referred to as heat vulcanizable silicone rubber compositions although they can be vulcanized at elevated temperatures.
The more traditional compositions to which this term applies comprises compositions which generally have the characteristics of having a diorganopolysiloxane gum as base polymer wherein the viscosity of the base polymer varies anywhere from 500,000 to 300,000,000 centipoise at 25.degree. C. Preferably such a diorganopolysiloxane gum also contains certain amount of vinyl in it for use in cross-linking. Such a polymer also contains filler which may be a reinforcing filler such as fumed silica, precipated silica or an extending filler such as iron oxide, zinc oxide, and so forth. Such compositions have various types of additives in them as do the RTV compositions which in addition to the basic ingredient to give the composition certain advantageous properties which are desired for a particular use. Accordingly, such heat vulcanizable silicone rubber compositions may have rare earth octoates in them so as to give the composition desired compression set properties. The filler, in particular, the reinforcing fillers such as fumed silica or precipated silica, give the composition a high tensile strength. There may be incorporated into the composition platinum as a flame retardant additive. There may be also incorporated into the composition a second vinyl polymer with a higher content of vinyl than the base polymer so as to give the composition high strength properties. An examples of such a composition is, for instance, to be found in Bobear U.S. Pat. No. 3,660,345 which is hereby incorporated by reference.
An addition cured composition which is inhibited is to be found in the patent of William J. Bobear U.S. Pat. No. 4,061,609 which is hereby incorporated by reference. An example of a room temperature one component RTV composition which a brief description was given above is to be found in Beers U.S. Pat. No. 4,100,129. An example of a two component RTV composition of which the above was a brief description, is to be found in Bessimer et al U.S. Pat. No. 3,888,815 which is hereby incorporated by reference. All the above patents were given as examples of different types of composition which cure to a silicone elastomer.
However, proceeding with a heat vulcanizable silicone rubber composition, in addition to the gum, filler and the additives, there is another necessary ingredient in this composition; that is, an organic peroxide. The organic peroxide is utilized to cure the composition. The organic peroxide when heated at temperatures above 100.degree. C. forms free radicals which attacks the diorganopolysiloxane gum and causes it to cross-link. Accordingly, since temperatures above 100.degree. C. are necessary to activate the organic peroxide catalyst, these compositions are simply referred to as heat vulcanizable silicone rubber compositions. There may also be added self-bonding additives to such heat curable silicone rubber composition. An example of such heat curable silicone rubber composition is to be found in U.S. Pat. No. 3,813,364 which is hereby incorporated by reference.
Irrespective of the various additives that are present in the composition, such heat vulcanizable or heat curable silicone rubber compositions are utilized usually in the case where high strength is desired. That is, the composition cures to a silicone elastomer of high tensile strength. Accordingly, when a composition of high tensile stength is desired with various other types of properties such as flame retardant, self-bonding and whatnot, there is utilized a heat curable silicone rubber composition. The only deterent from utilizing it, is that the composition has to be heated at elevated temperatures to set or vulcanize to a silicone elastomer. Accordingly, the basic ingredient in such heat curable silicone rubber composition is a diorganopolysiloxane gum. The process by which this diorganopolysiloxane gum is prepared will be gone into below. However, what is necessary to mention now is that by the process by which such diorganopolysiloxane gums are prepared it is normal in such gums that as much as 15 percent by weight are volatiles. Such volatiles are normally low boiling cyclopolysiloxane and low molecular weight linear siloxane compounds. It is undesirable that the composition be cured with a peroxide at elevated temperatures with such volatiles in a diorganopolysiloxane gum. If such volatiles are present in a diorganopolysiloxane gum when it is cured to a silicone elastomer not only will the composition undesirably be reduced in weight due to the evaporation of some of the volatiles, but also there will be a decrease in the desired tensile strength, then a desired elongation and the desired compression set of the cured composition. The desired physical properties of the cured silicone elastomer are lowered when the diorganopolysiloxane gum is cured with the volatiles present in it. Accordingly, it has become a practice to remove the volatiles from the diorganopolysiloxane gum prior to or during the incorporation of the filler in the composition and prior to curing to the composition with the organic peroxide. Normally such volatiles were removed by mixing the diorganopolysiloxane gum in a doughmixer along with silica filler wherein while the diorganopolysiloxane gum was being mixed with a filler and with a process aid present the gum was heated so that the volatiles were given off to the atmosphere. There was utilized in some case hoods for the removal of the volatiles from the doughmixers and the volatiles were vented to the atmosphere. However, with the advent of environmental restrictions this was not altogether possible.
In addition, processes are being developed for the utilization of such cyclics and low molecular siloxanes that is the volatiles for the production of polysiloxane polymers. Accordingly, it has become highly desirable to have an exacting and accurate process for removal of the volatiles from doughmixers so that they can be collected and either reused or incinerated or disposed as desired without contaminating the atmosphere of the workroom where the doughmixers are present. It is also desirable that the volatiles be removed and collected in as nearly pure form as possible such that they can be utilized in due course for the production of polysiloxane polymers.
Accordingly, it is one object of the present invention to provide a process for removing and collecting volatiles from a doughmixer in essentially pure form.
It is an additional object of the present invention to provide for a process for collecting volatiles in essentially pure form from a doughmixer in which a heat vulcanizable silicone rubber composition is being mixed in an expensive and efficient manner.
It is still an additional object of the present invention to provide a process for collecting the volatiles in an efficient and inexpensive manner from a Banbury mixing vessel in which there is mixed a heat vulcanizable silicone rubber composition.
It is yet an additional object of the present invention to provide for an efficient and inexpensive method for removing volatiles from a heat vulcanizable silicone rubber composition which is mixed in a Banbury mixer such that the volatiles can be segregated in an essentially pure form and without contamination of any effluent streams or the atmosphere.
These and other objects are accomplished by means of the disclosure set forth herein and below.